1. Field of the Invention
The present invention relates to a DC/AC inverter device, and, in particular, relates to a power-system-interconnected inverter device including a multilevel circuit.
2. Description of the Related Art
In recent years, for example, solar power generation systems have prevailed, and, from a viewpoint of the high efficiency thereof, non-insulated types have entered the mainstream of power-system-interconnected inverters (hereinafter, a power system is simply referred to as a “system”). So as to generate a substantially sinusoidal voltage in an insulated type inverter (so as to inject a substantially sinusoidal current into a system), an inverter device has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2006-223009, the inverter device being equipped with a multilevel circuit outputting a plurality of voltages whose number is greater than or equal to three.
In FIG. 1 in Japanese Unexamined Patent Application Publication No. 2006-223009, the configuration of a five-level inverter is disclosed where a series circuit including four capacitors and a series circuit including eight switch elements are provided between the positive and negative electrode terminals of a direct-current power supply and switch elements and diodes are connected between the connection points of these capacitors and the connection points of these switch elements.
When an inverter device cooperating with a system is configured, three sets of the single-phase inverter devices are provided in an inverter device connected to, for example, a system of a three-phase alternating current. In addition, so as to configure an inverter device connected to, for example, a household single-phase three-wire system, two sets of the single-phase inverter devices are provided.
However, if the number of levels is n, switch elements whose number is 2(n−1) are necessary for a multilevel circuit. Therefore, in particular, for household use, an increase in the cost of a semiconductor element or another passive component becomes a major issue.